Hi,
We have a design with the LTC6951 on a custom board with a reference clock of 25 MHz.
The target is a VCO frequency of 4800 MHz but the PLL is only able to lock in the frequency range of 4175 to 4350 MHz.
When it is set to 4800, it is not locked and the output frequencies are corresponding to 4350 MHz on the VCO and VTUNE is around 4V.
It seems like the calibration is not working, but it is manually triggered and AUTOCAL is enabled so it should trigger after registers 0x05 and 0x06. Also BD is 96 according to Fpfd = 25 MHz.
The reference clock flag is marked as OK. Below are the schematics and configuration of the PLL.
I am aware that the loop filter is not the optimum for the thermal noise in this configuration, I have modified it to get it close to the calculated in the software tool (for 1mA) and still the same behavior.
So what can be the reason is not locking outside of that frequency range?
Thanks in advance,
Marta
ltc6951_spi_write(0, LTC6951_REG_01, 0x3A); // STAT output config
ltc6951_spi_write(0, LTC6951_REG_02, 0x00); // Power downs and CAL trigger
ltc6951_spi_write(0, LTC6951_REG_03, 0x78); // AUTOCAL 1, RAO 0, BST 0, FLT 0
ltc6951_spi_write(0, LTC6951_REG_04, 0x70); // B 96
ltc6951_spi_write(0, LTC6951_REG_05, 0x04); // R 1
ltc6951_spi_write(0, LTC6951_REG_06, 0xC0); // N 192
ltc6951_spi_write(0, LTC6951_REG_07, 0x00); // CP 1mA
ltc6951_spi_write(0, LTC6951_REG_08, 0x00); // P 2
ltc6951_spi_write(0, LTC6951_REG_09, 0x30); //OUT0 OFF
ltc6951_spi_write(0, LTC6951_REG_0A, 0x03);
ltc6951_spi_write(0, LTC6951_REG_0B, 0x11); //OUT1 1200 MHz M1 2
ltc6951_spi_write(0, LTC6951_REG_0C, 0x00);
ltc6951_spi_write(0, LTC6951_REG_0D, 0x03); //OUT2 OFF
ltc6951_spi_write(0, LTC6951_REG_0E, 0x00);
ltc6951_spi_write(0, LTC6951_REG_0F, 0x16); //OUT3 100 MHz M3 24
ltc6951_spi_write(0, LTC6951_REG_10, 0x00);
ltc6951_spi_write(0, LTC6951_REG_11, 0x18); //OUT4 50 MHz M4 48
ltc6951_spi_write(0, LTC6951_REG_12, 0x00);
ltc6951_spi_write(0, LTC6951_REG_02, 0x01); // Trigger manual calibration
